To use a heavy gun with a small body type, first, change to a heavy body type. Choose a gun (make sure it's exactly how you want it as you cannot edit it afterward). Pick any gun of any weight. Now do the same for your secondary. Now make it your default weapon and go change back into a medium/light body type. Your default guns will stay the same, but your body will not, thus, when you play matches, you can have a light body type with a gatling gun.